Answer: For #11, he will be able to hear for 6 miles.
The equation of a circle ends with r^2 on the right sides. Right now, we have 36. We need to change this to 6^2, so we can identify the radius as 6.
For #12, you would need to put into into the equation of a circle. Then, you could identify the center.
It would be (x - 6)^2 + (y +3)^2 on the left side. Therefore, the center would be (6, -3).